Interpretation of the strengthening of steel with lower bainite and martensite mixed microstructure

A mixed microstructure consisting of lower bainite and martensite was introduced in an alloyed medium carbon steel (SCM 435) through an appropriate isothermal treatment and the variation in microhardness was measured as a function of lower bainite fraction. It was shown that a maximum hardness peak appeared when the steel contained about 20% lower bainite. A double etching technique using picric acid to reveal the prior austenite grain and Le Pera etchant to reveal the lower bainite phase showed that the microstructure became manifest through the partition of the prior austenite grain by the lower bainite plates. Based on such microstructural characteristics, a model interpreting the strengthening of the steel through the presence of lower bainite was constructed and shown to be well-fitted with the experimental results. The model took into consideration of three factors: the refinement of the martensitic substructure through the presence of lower bainite, the carbon shifting from bainitic ferrite into untransformed austenite during lower bainite transformation and the mixture rule.